Ultrasonic sensors are well known for the detection of the presence of a liquid. One such type of sensor includes a housing to be placed in the location where liquid presence is to be detected which holds a transmitting and receiving element, such as a piezoelectric crystal, separated by an air gap. An amplifier is in the path between the two crystal elements. When there is no liquid present in the gap, the sensor produces no signal. When liquid is present in the gap, the amplifier gain becomes greater than one, i.e. the circuit oscillates, and an output signal is produced to indicate the presence of the liquid.
It is also desired in some cases to determine the type of liquid, for example, discriminate between the presence of water, gasoline, oil or a combination thereof, so that appropriate action can be taken. A typical application is in a leak detector used in connection with double hull storage tanks at a gasoline station to determine if there is liquid leaking between the two walls of the tank and the type of liquid leaking.